In UMTS (Universal Mobile Telecommunications System) networks, for the purpose of improving spectral efficiency and further improving data rates, by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), it is performed exploiting maximum features of the system based on W-CDMA (Wideband Code Division Multiple Access). For the UMTS network, for the purpose of further increasing high-speed data rates, providing low delay and the like, Long Term Evolution (LTE) has been studied.
In the 3G system, a fixed band of 5 MHz is substantially used, and it is possible to achieve transmission rates of approximately maximum 2 Mbps in downlink. Meanwhile, in the LTE system, using variable bands ranging from 1.4 MHz to 20 MHz, it is possible to achieve transmission rates of maximum 300 Mbps in downlink and about 75 Mbps in uplink. Further, in the UMTS network, for the purpose of further increasing the wide-band and high speed, successor systems to LTE have been studied (for example, LTE Advanced (LTE-A)). For example, in LTE-A, it is scheduled to expand the maximum system band of 20 MHz in LTE specifications to about 100 MHz.
In addition, the MIMO antenna system has been proposed as radio communication techniques for transmitting and receiving data using a plurality of antennas and improving throughput and spectral efficiency (for example, see Non-patent Document 1). In the LTE-scheme system, two modes, spatial multiplexing transmission mode (SU-MIMO (Single User MIMO) and transmission diversity transmission mode, are specified as a downlink MIMO mode. The spatial multiplexing transmission mode is to spatially multiplex signals of a plurality of streams in the same frequency and time, and is effective at increasing the peak data rate. The transmission diversity transmission mode is to transmit a signal of the same stream subjected to space-frequency (time) coding from a plurality of antennas, and is effective at improving the reception quality at cell-edge users by transmission antenna diversity effect.
In such a MIMO antenna system, techniques (rank adaptation) have been proposed to control the number of layers for spatial multiplexing to be optimal corresponding to the reception status in each mobile station receiver. In this rank adaptation, based on the downlink channel information (reception SINR, fading correlation between antennas), the base station transmitter controls so as to perform information transmission in the spatial multiplexing transmission mode for mobile station receivers excellent in the channel state, while performing information transmission in the transmission diversity transmission mode for mobile station receivers poor in the channel state. In this rank adaptation, the mobile station transmitter transmits the number of layers for spatial multiplexing to the base station receiver as feedback information (rank information). This rank information is to affect greatly information transmission, and it is extremely important that the rank information is suitably transmitted to the base station receiver as feedback.